The Economist explains

How do you stop an asteroid hitting Earth?

MOST asteroids in the solar system come nowhere near the Earth, and of those that do, most are so small that they burn up harmlessly in the atmosphere. But, as a report written by America's National Academy of Sciences points out, a big one is almost certain to hit eventually. Depending on how big it is, and where it comes down, it could wipe out a city, a country or possibly even human civilisation. In February the world got a graphic reminder of the risk, when a small rock disintegrated above the Russian city of Chelyabinsk, killing nobody but injuring around 1,500 people. That has focused attention on how to protect the Earth from future impacts, because asteroid strikes are one of the few natural disasters that humans have the power to prevent. But how?

Even small asteroids are massive and very fast-moving, so destroying them outright is not a realistic option. Instead, scientists aim to nudge them off course, so that they sail harmlessly past the Earth. To do that, though, they must find them first. NASA, America's space agency, reckons it has found almost all the potentially Earth-threatening asteroids with a diameter of a kilometre or more (big enough to cause global devastation). In 2005 it was given the task of finding 90% of those above 140 metres wide. Other space agencies run similar programmes, and the B612 Foundation, a charity, aims to launch an asteroid-hunting space telescope in 2016. But the smaller an asteroid is, the harder it is to spot. Worse, small ones are more common than big ones, and they can still be dangerous despite their size. Although it was only around 30 metres across, the Chelyabinsk meteorite packed about the same explosive power as a medium-sized nuclear bomb. A telescope being built at the University of Hawaii aims to scan the night sky looking for small rocks that have evaded detection, the idea being to give the authorities enough time to evacuate the target area.

Assuming that a dangerous rock is found, though, there are plenty of options. The one that springs to most people's minds—possibly because it was employed in the films "Deep Impact" and "Armageddon"—is simply to blast the rock with nuclear weapons. That is one of the options being studied by NEOShield, a European Union project that is researching asteroid-deflection. But it is risky. Many asteroids are simply clumps of rock held loosely together by gravity. A nuclear blast might simply split a big asteroid into two medium-sized ones, either or both of which could still hit the Earth. Provided an asteroid is spotted with plenty of time to spare (years or decades, depending on its size), subtler methods may therefore be more reliable. One would be to use a "kinetic impactor", astronomer-speak for ramming the rock with something heavy to knock it off course. An asteroid might also be gently pushed into a less dangerous orbit using a solar sail, a rocket motor or an ion engine. Most graceful of all would be a "gravity tractor", in which a spaceship is flown near the asteroid. Its minuscule gravitational influence would then perturb the rock's orbit enough to ensure that it missed the Earth.

Even after Cheylabinsk, talking about the risk from asteroids is likely to provoke giggles at dinner parties. But with well over a hundred known impact craters on Earth alone, spending a few tens of millions of dollars on asteroid defence looks like a sensible insurance policy. For the truly paranoid, though, there are some space-borne risks that are impossible to insure against. Dangerous asteroids have orbits that frequently bring them close to the Earth, which makes them comparatively easy to spot. That is not true of comets, which zip in and out of the inner solar system on highly elongated orbits. In 1994 the comet Shoemaker-Levy 9 slammed into Jupiter, leaving an Earth-sized hole in that planet's atmosphere. If such a long-period comet came zooming out of deep space on a collision course with Earth, astronomers would probably have too little warning to do anything about it. The risk of that happening is virtually zero. You are more likely to die in almost any other way you can think of. But if it ever did happen, it really would be Armageddon.

I knew that the effects of the eruption of Mount Tambora were considerable. And yes the eruption of 1815 disrupted the cliamte in 1816 ('the year without a summer' indeed).
But it would be truly remarkable if it had any impact impact at all on Napoleon's march to Moscow. Given that was in 1812.
Unless volcanic debris also travels in time?

Sound requires a medium (such as air) in which to travel. So this would be tricky. Also, the main drawback of using nukes on an asteroid is not contamination, but the risk of turning one dangerous rock into several.

I thought that the size of the rock had a logarithmic relation to its damage. So reducing it in bits will result in much less "overall" damage as more surface would be exposed to atmospheric friction. Most asteroids after all don't even make it to the surface.

Chances are before any huge asteroid or comet could smash our earth, human beings might have been completely obliterated from the surface of the planet, except some lucky ones who managed to escape to space earlier.
Humans are extremely self-destructive, just face the inevitable destiny calmly. (btt1943)

Bit of a common misconception here - you couldn't use a nuclear bomb to 'blow an asteroid up'. There's no gas about to do the 'blowing'. Chemical explosives generate a large volume of gas very rapidly (so they'd work in a vacuum) but nukes just dump a huge amount of heat into their surroundings. On Earth, this makes the air around the device super-heated, so it explodes, creating the blast wave. In space though there is no gas to exert this force. You'd just briefly make the target very hot. This could actually be a terrible move, as you might end up fusing a loose collection of rock together - turning a wad of buck-shot into a rifle bullet. Whoops...

If global warming is heating up the earth.
One supervolcano eruption like Mt. Tambora in 1816 resulted in a world wide cloud cover that blocked out the Sun, caused global winter, caused a year without summer, wiped out Napoleon's Grand Army march to Russia, froze Josephine's wine in Versaille, exterminated hundreds of animal species, killed milllions, and nearly ended humanity. See the 'Year without Summer'.

One large meteor caused a cloud that blocked the sun causing global winter that exterminated the dinosaurs.
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WE flagellate ourselves over man made global warming.
But global cooling may be one brief event away,
and many of us will not survive it.

We should fear Global Cooling as much as Global Warming.
We are Godilocks--we need things just right.

And often fate cannot be changed, but must be endured.

There are some things we can change.
And many more that we cannot.
And wisdom is knowing the difference.